Three new chalcogenides namely NaGaGe3Se8, K3Ga3Ge7S20, and K3Ga3Ge7Se20 of A-Ga-Ge-Q (A = Na, K; Q = S, Se) system were obtained for the first time. They crystallize in two different new structures, albeit both in the monoclinic space group P21/c. NaGaGe3Se8 has a layered structure consisting of two dimensional [M4Se8]− layers separated by Na+ cations, while the structures of K3Ga3Ge7Q20 (Q= S, Se) are constructed by the incompletely isolated quasi-2D [M10Q21]5− layers, leading to large channels loosely occupied by K+ cations. Interestingly, thermal analysis indicates that the three title compounds are all congruent-melting compounds, which is uncommon for quaternary compounds and makes bulk crystal growth by Bridgeman technique possible. UV-vis-NIR spectroscopy measurements reveal that the optical band gaps of the three compounds are 2.35, 3.25, and 2.23 eV respectively. In addition, the electronic structure calculation on NaGaGe3Se8 shows that the band gap mainly determined by the GaSe4 and GeSe4 groups. 

Dalton Transactions, 2015